Thermally induced reflectivity switch for laser thermal processing

ABSTRACT

A method, apparatus and system for controlling the amount of heat transferred to a process region ( 30 ) of a workpiece (W) from exposure with laser radiation ( 10 ) using a thermally induced reflectivity switch layer ( 60 ). The apparatus of the invention is a film stack ( 6 ) having an absorber layer ( 50 ) deposited atop the workpiece, such as a silicon wafer. A portion of the absorber layer covers the process region. The absorber layer absorbs laser radiation and converts the absorbed radiation into heat. A reflective switch layer ( 60 ) is deposited atop the absorber layer. The reflective switch layer may comprise one or more thin film layers, and preferably includes a thermal insulator layer and a transition layer. The portion of the reflective switch layer covering the process region has a temperature that corresponds to the temperature of the process region. The reflectivity of the reflectivity switch layer changes from a low reflectivity state to a high reflectivity state at a critical temperature so as to limit the amount of radiation absorbed by the absorber layer by reflecting the incident radiation. This, in turn, limits the amount of heat transferred to the process region from the absorber layer.

This is a continuation divisional of application(s), Ser. No.09/592,184, filed on Jun. 12, 2000, now U.S. Pat. No. 6,303,476.

FIELD OF THE INVENTION

The present invention relates to laser thermal processing, and inparticular to a method of and apparatus for delivering precise amountsof thermal energy to a workpiece to be so processed.

BACKGROUND OF THE INVENTION

Laser thermal processing (LTP) is used to process workpieces such assemiconductor wafers in the manufacturing of semiconductor devices. Suchprocessing allows for the fabrication of transistors with very low sheetresistance and ultra-shallow junctions, which results in a semiconductordevice (e.g., an integrated circuit or “IC”) having higher performance(e.g., faster speed).

One method of LTP applied to semiconductor manufacturing involves usinga short-pulsed laser to thermally anneal the source and drain of thetransistor and to activate the implanted dopants therein. Under theappropriate conditions, it is possible to produce source and drainjunctions with activated dopant levels that are above the solidsolubility limit This produces transistors with greater speeds andhigher drive currents. This technique is disclosed in U.S. Pat. No.5,908,307 entitled “Fabrication Method for Reduced Dimension FETDevices,” incorporated by reference herein.

It is expected that ICs will benefit from the performance improvementdemonstrated with performing LTP on single transistors. Unfortunately,scaling LTP from single transistor fabrication to full integratedcircuit fabrication is difficult. The LTP process has a very narrowprocess window (i.e., the range in laser energy that activates thetransistor without causing damage is narrow) and requires considerableuniformity, stability and reproducibility in the absolute energydelivered to (and absorbed by) each transistor.

Modem ICs contain a variety of device geometries and materials, and thusdifferent thermal masses. To achieve uniform performance in eachtransistor, it is necessary that all transistors be heated (annealed) toessentially the same temperature. This places constraints on thepermissible range of laser energy delivered to each transistor in thecircuit. As a result, two problems arise. The first is that it isdifficult to achieve sufficiently uniform exposures (both spatially andtemporally) to accomplish uniform heating. The second is that differentdevice geometries require different amounts of incident laser energybecause their different thermal masses will affect the local temperaturein the doped regions (junctions).

Of these two problems, the more daunting is the effect of localtransistor density. Most modem integrated circuits have a variety oftransistor densities across the circuit. This variation has two effectson the LTP process. The first is that the local reflectivity variesspatially, thereby changing the amount of heat locally absorbed evenwith uniform illumination. The second is that the local thermal massvaries spatially. A larger thermal mass requires greater absorbed laserenergy to reach the required annealing temperature. As a result, achange in the local thermal mass requires a change in the amount oflaser energy absorbed that is required to produce proper annealing. Evenwith perfectly uniform illumination, there can be significanttemperature variations between different transistors on a single IC, orbetween ICs. This leads to undesirable variations in transistorperformance across a single IC and across a product line.

In principle, it may be possible to compensate for the location ofhigher transistor density across the device by providing a tailoredexposure having increased laser fluence in the higher density regions.However, this would require knowing the precise circuit layout acrossthe device for each device to be processed, and would also requireprecise tailoring of the spatial irradiance distribution of the exposureto match the circuit layer. This endeavor, if it could be accomplishedat all, would involve complex apparatus and significant expense.

SUMMARY OF THE INVENTION

The present invention relates to laser thermal processing, and inparticular to a method of and apparatus for delivering precise amountsof thermal energy to a workpiece to be so processed.

The present invention solves the problem of non-uniform thermal heatingof a workpiece processed using laser radiation by introducing athermally-induced reflectivity “switch” that controls the amount of heattransferred to a workpiece, such as a silicon wafer. This reflectivityswitch layer comprises one or more layers of material designed such thatthe reflectivity of the switch to incident laser radiation changes from“low” to “high” as one or more underlying process regions of theworkpiece reach a predetermined temperature. This temperature may be,for example, the temperature at which the process region is activated.For example, the one or more underlying regions may be the source anddrain regions of a transistor or a doped region of a junction, and thepredetermined temperature may be the activation temperature of theprocess region. The portions of the reflective switch layer overlyingthe process regions switches from a low reflectivity state to a highreflectivity state and reflects additional incident laser radiation whena critical switch temperature is achieved, thereby preventing furtherheating of the underlying process regions and limiting the temperatureof the one or more underlying regions to a maximum value.

When the present invention is applied to semiconductor manufacturing andforming IC devices having transistors, the predetermined temperature isthat where amorphous silicon in the source-drain regions of thetransistors reach a temperature between 1100 and 1410° C. At this point,the amorphous silicon is melted and the dopants become activated. Thistemperature is low enough so that the underlying crystalline siliconsubstrate does not melt, which is desirable from the viewpoint of deviceperformance. The reflectivity switch of the present invention preventslocal regions on the wafer from heating substantially beyond thepredetermined temperature due to a variety effects, such as fluctuationsin the laser energy, the spatial uniformity of the laser beam, or thethermal mass variations due to the transistor density.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional diagram of the reflective switchof the present invention shown as part of a film stack arranged on asemiconductor wafer having an amorphous doped region, with the waferarranged in a wafer holder in relation to a laser light source;

FIG. 2 is the same as FIG. 1, but the reflective switch layer of thefilm stack comprises a layer of silicon dioxide adjacent the absorberlayer, and amorphous or polycrystalline silicon adjacent the silicondioxide layer;

FIG. 3 is a plot of the temperature T vs. time for the temperature (T₆₄)of the reflective switch layer and the temperature (T₃₀) of theamorphous doped region versus time, showing the point, T_(c), where thereflectivity of the switch layer transitions from a low reflectivitystate (i.e., transparent state) to a high reflectivity state;

FIG. 4 is a plot of reflectivity R versus time for the reflective switchlayer, showing the transition from a low reflectivity state (i.e.,nearly transparent state) to a high reflectivity state; and

FIG. 5 is a cross-sectional schematic diagram of a wafer having devices(e.g., transistors) in a region of high device density and a region oflow device density, with the film stack of FIG. 1 arranged thereon.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to laser thermal processing, and inparticular to a method of and apparatus for delivering precise amountsof thermal energy to a workpiece to be so processed.

The basic concept of the reflectivity switch is illustrated in FIGS. 1and 2 with regard to processing a semiconductor substrate as part of theprocess of manufacturing a semiconductor device such as a junction or atransistor. In FIG. 1, there is shown a film stack 6 formed on a siliconsemiconductor wafer W as a workpiece to be processed using LTP and laserirradiation 10 from a laser light source L. Wafer W is supported by awafer support member WS such that light source L, film stack 6 and waferW all lie along an axis A, as shown in FIG. 1. Laser radiation 10 ispreferably pulses of light having a wavelength of between 500 nm and1100 nm. A suitable laser light source L includes a YAG laser operatingat 1064 nm, a frequency-doubled YAG laser operating at 532 nm, and anAlexandrite laser operating between 700 and 800 nm. Suitable laser pulselengths range from 1 nanosecond to 1 μsecond, and suitable energy levelsrange from 0.1-10 J/cm².

Wafer W comprises a crystalline silicon region 20 within which is formedan amorphous doped silicon region 30 having dopants 34. For the sake ofexplanation, amorphous doped region 30 is considered as a single dopedregion. However, amorphous doped region 30 represents one example of aregion to be processed, referred to herein as a “process region.” Forexample, wafer W may contain a plurality of amorphous doped regions 30,or one positively doped region and one negatively doped region servingas source and drain regions, respectively, of a transistor.

With continuing reference to FIG. 1, amorphous doped region 30 may beformed by performing an ion implant of Si or Ge ions into wafer W to atarget depth ranging from a few angstroms to about 1000 angstroms. Thisimplantation process disorders the substrate crystal structure incrystal region 20 to the point of making this implanted regionamorphous. The implanted species can be Si, Ge, Ar, As, P, Xe, Sb, andIn. Implantation of amorphizing dopants can be performed with knownapparatus, such as the 9500 XR ION IMPLANTER™, commercially availablefrom Applied Materials, Inc., San Jose, Calif.

A second dopant ion implant is then performed using p-type dopant ions(e.g., boron, aluminum, gallium, beryllium, magnesium, or zinc) orn-type dopant ions (e.g., phosphorous, arsenic, antimony, bismuth,selenium, and tellurium) from an ion implanter. The ions are acceleratedto a given energy level (e.g., 200 eV to 40 KeV) and implanted in thepreviously amorphized region to a given dose (e.g., about 1×10¹⁴atoms/cm² to 1×10¹⁶ atoms/cm²), thereby forming doped, amorphous region30. The latter typically has, in practice, a concentration of dopantthat is graded with depth into wafer W. The first and second steps ofthe present embodiment can be interchanged to achieve the same effect,or carried out in a single step if the dopant implant also amorphizescrystalline region 20.

Deposited atop amorphous silicon region 30 is an absorber layer 50comprising a material capable of absorbing incident laser radiation andconverting the absorbed radiation into heat. Absorber layer 50 heeds tobe capable of withstanding high temperatures, i.e., temperatures inexcess of the crystalline silicon melting temperature of 1410° C. Thematerial making up absorber layer 50 must also be easily removablewithout impacting the layers or regions below. One role of absorberlayer 50 is to maintain the physical structure of devices resident in oron wafer W during processing. An exemplary material for absorber layer50 is tantalum nitride (TaN), deposited to a thickness of between 500and 1000 angstroms via sputtering or by CVD. Other preferred materialsfor absorber layer 50 include titanium (Ti), titanium nitride (TiN),tantalum (Ta), tungsten nitride (WN), silicon dioxide, silicon nitride,or a combination of these. A silicon dioxide or silicon nitride layermay need to be deposited as part of the absorber layer to preventcontamination of wafer W by the absorber layer material (i.e., betweenmetal and semiconductor), or adjust the reflectivity of the absorberlayer.

A thin strippable layer 40 is optionally placed between absorber layer50 and amorphous silicon region 30 to facilitate stripping of theabsorber layer after LTP is performed. Exemplary materials for strippinglayer 40 include silicon dioxide and silicon nitride, which can bedeposited by sputtering or by CVD.

Further included in film stack 6 is a reflectivity switch layer 60formed atop absorber layer 50. Layer 60 is designed so that it isinitially substantially transparent to laser radiation 10, allowingabsorber layer 50 to perform as described above. However, the propertiesof layer 60 are such that its reflectivity to incident laser radiation10 changes from low to high when it reaches a certain temperature,referred to herein as the threshold temperature.

Reflectivity switch layer 60 can comprise a single film layer ormultiple film layers (i.e., one or more film layers). With reference toFIG. 2, in one embodiment, reflective switch layer 60 comprises a firstthermal insulating layer 62 of silicon dioxide and a second transitionlayer 64 of amorphous or polycrystalline silicon atop the silicondioxide layer. It is desirable to design the thicknesses of reflectivityswitch layer 60 so as to optimize the coupling of the laser radiation 10into absorber layer 50. This can be done by using standard thin filmdesign techniques to optimize the thicknesses and index of refraction ofthe materials in film stack 6 such that there is a minimum reflectivityat room temperature for incident radiation 10. In a preferred embodimentof the present invention, layer 62 has a thickness ranging from about10-250 nm, while the thickness of layer 64 ranges from about 10-250 nm.This provides a reflectivity in the low reflectivity state in the rangefrom about 5% to 20%, and a reflectivity in the high reflectivity statein the range from about 50% to 75% for a wavelength of light of about1000 nm.

Method of Operation

The present invention operates as follows. With reference to FIGS. 1 and2, LTP of wafer W is performed by directing laser radiation 10 to filmstack 6 along an axis A for the purpose of activating amorphous dopedregion 30. Reflectivity switch layer 60 is initially substantiallytransparent. Accordingly, most of laser radiation 10 passes throughlayer 60 and is incident absorber layer 50. Radiation 10 is absorbed inlayer 50, thereby heating this layer. Absorber layer 50 heats up andre-radiates this heat to amorphous doped region 30 and to reflectivityswitch layer 60. Doped amorphous region 30 is thus heated to itsactivation temperature of between 1100-1410° C., while reflective switchlayer 60 is also heated to its critical temperature. In the activationtemperature range, dopants 34 become incorporated into the lattice sitesand are “activated.” However, if too much laser radiation is incidentabsorber layer 50 then amorphous region 30 is provided with too muchheat. In this regard, the present invention prevents the workpiece(wafer W) from reaching or exceeding a maximum workpiece temperature,which is an upper temperature beyond which there is an undesirableaffect on the workpiece (e.g., melting). This extra heat can cause theunderlying crystalline silicon region 20 to melt. This is undesirablebecause it can adversely affect the properties of amorphous doped region30. Where the latter constitutes the source or drain region of atransistor, such overheating can damage the transistor gate region (notshown).

FIG. 3 illustrates the temperature T₃₀ of amorphous doped region 30during the LTP annealing process as described above. Temperature T₃₀rises as a function of time during LTP exposure. Unconstrained,temperature T₃₀ rises above the melting point T_(p)=1410° C. forcrystalline silicon, as illustrated with a dotted line D. However, withreflectivity switch layer 60 present (see FIG. 1), the temperature T₆₄of reflectivity switch layer 64 tracks temperature T₃₀ of region 30.Accordingly, reflectivity switch layer 64 can be designed to have atemperature that is greater than or less than temperature T₃₀ byadjusting the thickness and thermal characteristics of layer 62. Forexample, where reflectivity switch layer comprises two layers 62 and 64as discussed above, this may involve adjusting the thickness of layer 62in the manner described in detail below. In FIG. 3, the criticaltemperature T_(c) is set such that this temperature is reached when thetemperature T₃₀ of process region 30 reaches temperature T_(p). However,it will often be preferable to set temperature T_(c) so that it isreached prior to when the temperature T₃₀ reaches T_(p).

When reflectivity switch layer 64 reaches its critical temperatureT_(c), the reflectivity switches from a low reflectivity state R_(L) toa high reflectivity state R_(H), as illustrated in FIG. 4. The switchoccurs primarily because of the change in reflectivity of layer 64 whenit reaches this critical temperature (such as when the material changesfrom a solid to liquid state). The timing, or tracking, of thetemperature of layer 64 relative to T₃₀ is accomplished by adjusting thethermal conductivity and thickness of layer 62. Properly designed,reflectivity switch layer 60 can have a low reflectivity (less than 10%)and a high reflectivity (>70%).

Reflectivity switch layer 60 is designed as follows: The process beginsby choosing the operational laser wavelength and pulse-length. For thisexample, consider a wavelength of 1064 nm and a pulse-length of 10nanoseconds. Next is chosen optional strippable layer 40 and absorberlayer 50. Typically, strippable layer 40 can be 10-20 nm of silicondioxide or silicon nitride, and absorber layer 50 can be 20-100 nm oftitanium, titanium-nitride, titanium, or a combination of these layers.The purpose of absorber layer 50 is to absorb incident laser radiation10, so sufficient material must be used to absorb greater than about 75%of the incident radiation. For this example, a 10 nm oxide for layer 40and 40 nm titanium for layer 50 is a suitable choice. Next, an arbitrarythickness for layer 62 is chosen. Appropriate materials are eithersilicon dioxide or silicon nitride. For this example, 50 nm of silicondioxide is a suitable choice. Finally, an arbitrary thickness for layer64 is chosen. Appropriate materials for layer 64 are any materials thatexhibit a significant change in reflectivity when heated to atemperature range between about 1000-3000° C., such as crystallinesilicon, polycrystalline silicon, amorphous silicon, or titanium. Layer64 is chosen such that its optical properties change significantly whenit melts. A layer 64 comprising 100 nm of amorphous silicon is asuitable choice for the present example.

The next step in designing reflectivity switch layer 60 is to minimizethe optical reflectivity of film stack 6 using a thin-film analysiscode. Several such codes are commercially available, such as CODE V fromOptical Research Associates, Calif. The reflectivity of film stack 6 isminimized from the stack by adjusting layer 64, the 100 nm of amorphoussilicon. The goal is to produce a film stack 6 with a reflectivity lessthan 10%. Once this is accomplished, a thermal transport code is used,such as TOPAZ from Lawrence Livermore National Laboratory, Livermore,Calif., to calculate the thermal properties of film stack 6 and theunderlying layer 30. In particular, the temperature of layer 64 relativeto region (layer) 30 is calculated and plotted. The thickness of layer62 is then varied until layer 64 reaches its melt temperature at thesame time when region 30 reaches its activation temperature. Thisinsures that layer 62 will begin to reflect any additional laserradiation away from the structure after region 30 has been activated.Finally, the reflectivity of the stack is re-optimized (by optimizinglayer 64) with the new thickness value for layer 62. In the aboveexample, the optimum stack is calculated to be:

Layer 40: silicon dioxide: 10 nm

Layer 50 titanium: 40 nm

Layer 62: silicon dioxide: 80 nm

Layer 64: amorphous silicon: 163 nm

With this stack of materials, film stack 6 has a minimum reflectivity of6% (at room temperature), and a maximum reflectivity of 70% (at region30 activation temperature) is predicted.

Other examples of film stack 6 are as follows:

At a wavelength of 1064 nm and a pulse-length of 10 nsec:

Layer 40: silicon dioxide: 10 nm

Layer 50 titanium: 20 nm followed with titanium nitride: 20 nm

Layer 62: silicon dioxide: 80 nm

Layer 64: amorphous silicon: 163 nm

At a wavelength of 1064 nm and a pulse-length of 30 nsec:

Layer 40: silicon dioxide: 10 nm

Layer 50 titanium: 20 nm followed with titanium nitride: 20 nm

Layer 62: silicon nitride: 266 nm

Layer 64: amorphous silicon: 50 nm

Accordingly, reflectivity switch layer 60 is designed so it reaches itscritical temperature at which the reflectivity change occurs beforeamorphous doped region 30 reaches a temperature of about 1410° C., butafter it reaches the dopant activation temperature of 1100° C. This isachieved by properly designing thermal insulating layer 62, as describedabove. By choosing its thickness and thermal properties in the mannerdescribed above the temperature of transition layer 64 can be engineeredso that its reflectivity switches at the proper temperature. Oncereflectivity switch layer 64 transitions from a low reflectivity stateR_(L) to a high reflectivity state R_(H), incident laser radiation 10 isreflected, as indicated by reflected radiation 10′ in FIG. 2. Thisprevents further heating of absorber layer 50 and therefore, furtherheating of amorphous doped region 30.

By way of example, consider the two-layer reflectivity switch layer 60discussed above in connection with FIG. 2. When layer 64 reaches itsmelt temperature of 1100° C., it will begin to reflect a significantamount of incident laser radiation 10, as indicated by reflectedradiation 10′. The role of layer 62 is to provide the necessaryrelationship between the temperature of amorphous layer 30, and layer64. Accordingly, by tailoring the thickness of layer 62 in the mannerdescribed above, the temperature at which layer 64 “switches” relativeto when amorphous doped region 30 is activated can be controlled. Eventhough reflectivity switch layer 60 may begin to reflect radiation whenit reaches the switching temperature (e.g., 1100° C. for an amorphoussilicon), amorphous doped region 30 may be at a significantly differenttemperature. Generally speaking, reflectivity switch layer 60 isdesigned to change reflectivity state so as to allow activation of theprocess region without melting the surrounding region (e.g., crystallineregion 20).

Note also that for a reflectivity switch layer 60 comprising multiplelayers, only one of the layers may be the layer that changesreflectivity (i.e., the “transition layer”), while the other layers are“temperature-adjusting layers” that are used to set the criticaltemperature of the transition layer. For the two-layer example ofreflectivity switch layer 60 comprising layers 62 and 64, layer 64 isthe transition layer, while layer 62 is the temperature-adjusting layer.

Other possible compositions for reflectivity switch layer 60 include atwo-layer geometry with layer 62 comprising silicon dioxide, siliconnitride, silicon oxynitride, or any combination thereof, and layer 64comprising silicon, titanium or any other material that changesreflectivity state in the temperature range from 1000-3000° C. Thesefilms may be deposited by physical or chemical vapor deposition.

With reference now to FIG. 5, non-uniformities in laser radiation 10 orvariations in the density of devices 100 across wafer W influence thetemperature of amorphous doped regions 30, which in FIG. 5 are sourcesand drains 110S and 110D in devices 100. This will influence thetemperature of reflectivity switch layer 60. As a result, reflectivityswitch layer 60 will only activate when source and drain regions 110Sand 110D reach the dopant activation temperature range of 1100-1410° C.The density of devices 100 in region 120 is less than that of region130, so that region 120 has a smaller thermal mass as compared to region130. Accordingly, devices 100 in region 120 will be heated more quicklythan the devices in region 130.

As a result, when irradiated with laser radiation 10, devices 100 inregion 120 will reach their activation temperature before the devices inregion 130. Thus, portion 150 of reflectivity switch layer 60 lyingabove region 120 will transition to the reflective state first, and willreflect incident radiation 10. Meanwhile, devices 100 in region 130 takelonger to reach the activation temperature and continue to absorb heatfrom absorber layer 50. Accordingly, portion 160 of reflectivity switchlayer 60 lying above region 130 remains transparent for a longer timeand then transitions to the high reflective state when devices 100 inregion 130 reach their activation temperature. The same phenomenonoccurs where regions 120 and 130 have different reflectivities.

Because of the adaptive properties of reflectivity switch layer 60, itis difficult to overexpose regions (e.g., regions 120 and 130) on waferW having different thermal masses, or different reflectivities.Accordingly, locations where the local device geometry is such thatgreater or lesser amounts of laser radiation are required are readilyand automatically compensated.

Method of Forming a Semiconductor Device

Based on the above, the present invention includes a method of forming asemiconductor device from a semiconductor wafer. With reference again toFIG. 5, the method includes the steps of forming one or more processregion in semiconductor wafer W comprising devices 100 having amorphousdoped silicon regions, such as source and drain regions 110S and 110D,respectively, each having an activation temperature. The next stepsinvolve depositing an absorber layer over the process region, depositinga reflective switch layer atop the absorber layer, and irradiating theabsorber layer through the reflective switch layer to heat the absorberlayer and the reflective switch layer. These steps are described above,as is the step of heating the process region with heat from the absorberlayer until the reflective switch layer reaches the activationtemperature. At this point, the reflective switch layer switches to ahigh reflectivity state, thereby reducing the amount of radiationincident the absorber layer. The final step is then removing theabsorber layer and the reflective switch layer. This can be achieved byusing commercial etch techniques.

While the present invention has been described in connection withpreferred embodiments, it will be understood that it is not so limited.On the contrary, it is intended to cover all alternatives, modificationsand equivalents as may be included within the spirit and scope of theinvention as defined in the appended claims.

What is claimed is:
 1. A thermally induced reflectivity switch apparatusfor controlling the amount of heat transferred to a process region of aworkpiece from exposure with laser radiation, comprising: a) an absorberlayer deposited atop the workpiece, a portion of which covers theprocess region, wherein the absorber layer absorbs laser radiation andconverts the absorbed radiation into heat; and b) a reflective switchlayer deposited atop the absorber layer, wherein a portion of saidreflective switch layer covering said process region has a temperaturethat corresponds to the temperature of the process region and changesreflectivity from a low reflectivity state to a high reflectivity stateat a critical temperature so as to limit the amount of radiationabsorbed by the absorber layer and thus the amount of heat transferredto the process region.
 2. The apparatus of claim 1, wherein said processregion has an activation temperature and the workpiece has a maximumtemperature, and wherein said critical temperature is such that theprocess region temperature reaches said activation temperature but doesnot exceed said workpiece maximum temperature.
 3. The apparatus of claim1, further including a strippable layer arranged between said absorberlayer and said workpiece.
 4. The apparatus of claim 1, wherein theprocess region comprises a source and drain region of a transistor. 5.The apparatus of claim 1, wherein the process region comprises dopedamorphous silicon.
 6. The apparatus of claim 2, wherein the processregion comprises doped amorphous silicon.
 7. The apparatus of claim 6,wherein said activation temperature is between 1100 and 1410° C.
 8. Theapparatus of claim 1, wherein said absorber layer comprises at least oneof titanium, tungsten, tantalum, silicon oxide, silicon nitride, ortitanium-nitride.
 9. An apparatus according to claim 1, wherein saidreflectivity switch layer comprises a first layer of silicon dioxideadjacent said absorber layer and a second layer of amorphous silicon orpolycrystalline silicon adjacent said silicon dioxide layer.
 10. Anapparatus according to claim 9, wherein said first layer of silicondioxide has a thickness between 10-250 nm, and said second layer isamorphous silicon having a thickness between 10-250 nm.
 11. An apparatusaccording to claim 9, wherein said first layer of silicon dioxide has athickness between 10-250 nm, and said second layer is polycrystallinesilicon having a thickness between 10-250 nm.
 12. A system forcontrolling the amount of heat transferred to a process region of aworkpiece, comprising: a) a laser source; b) a workpiece support memberarranged adjacent the laser source for supporting the workpiece; and c)a thermally induced reflectivity switch apparatus arranged on theworkpiece comprising: i) an absorber layer deposited atop the workpiece,a portion of which covers the process region, wherein the absorber layerabsorbs laser radiation and converts the absorbed radiation into heat;and ii) a reflective switch layer deposited atop the absorber layer,wherein a portion of said reflective switch layer covering said processregion has a temperature that corresponds to the temperature of theprocess region and changes reflectivity from a low reflectivity state toa high reflectivity state at a critical temperature so as to limit theamount of radiation absorbed by the absorber layer and thus the amountof heat transferred to the process region.
 13. A system according toclaim 12, wherein said laser source emits laser radiation at awavelength between 500 nm and 1100 nm.
 14. A reflectivity switchapparatus, comprising: an absorber layer formed atop a workpiece andcovering one or more process regions; a reflectivity switch comprisingone or more layers formed atop the absorber layer, wherein saidreflectivity switch changes reflectivity from a low reflectivity stateto a high reflectivity state upon reaching a critical temperature. 15.The apparatus of claim 14, wherein the critical temperature correspondsto an activation temperature of the one or more process regions.
 16. Theapparatus of claim 14, wherein the critical temperature is less than amelting temperature of the workpiece.
 17. The apparatus of claim 16,wherein the workpiece comprises crystalline polysilicon and the criticaltemperature is less than 1410° C.
 18. The apparatus of claim 17, whereinthe one or more process regions include amorphous doped silicon.
 19. Theapparatus of claim 14, wherein the absorber layer comprises one or morematerials selected from the group of materials consisting of: titanium,titanium nitride, tantalum, tantalum nitride, tungsten nitride, silicondioxide, and silicon nitride.
 20. The apparatus of claim 14, wherein thereflectivity switch comprises a thermal insulating layer and atransition layer.
 21. The apparatus of claim 20, wherein the thermalinsulating layer comprises silicon dioxide.
 22. The apparatus of claim21, wherein the transition layer comprises amorphous or polycrystallinesilicon atop the silicon dioxide layer.
 23. The apparatus of claim 20,wherein the thermal insulating layer has thickness ranging from about 10to 250 nm.
 24. The apparatus of claim 20, wherein the transition layerhas a thickness ranging from about 10 to 250 nm.
 25. The apparatus ofclaim 14, wherein the reflectivity switch is adapted to provide at aselect wavelength a minimum amount of reflectivity below the criticaltemperature and a maximum amount of reflectivity above the criticaltemperature.
 26. The apparatus of claim 16, wherein the reflectivityswitch critical temperature is below the meting temperature of the oneor more process regions.
 27. The apparatus of claim 14, wherein in thereflectivity switch is solid in the low reflectivity state.
 28. Theapparatus of claim 27, wherein the reflectivity switch is liquid in thehigh reflectivity state.
 29. The apparatus of claim 14, furtherincluding a strippable layer between the workpiece and the absorberlayer.
 30. The apparatus of claim 29, wherein: the strippable layercomprises silicon dioxide; the absorber layer comprises titanium; andthe reflective switch layer comprises a thermal insulating layer ofsilicon dioxide and a transition layer of amorphous silicon.
 31. Theapparatus of claim 30, wherein the strippable layer of silicon dioxideis about 10 nm thick.
 32. The apparatus of claim 31, wherein theabsorber layer of titanium is about 40 nm thick.
 33. The apparatus ofclaim 32, wherein the thermal insulating layer is about 80 nm thick. 34.The apparatus of claim 33, wherein the transition layer of amorphoussilicon is about 163 nm thick.
 35. The apparatus of claim 34, whereinthe absorber layer also includes a layer of titanium nitride.
 36. Theapparatus of claim 35, wherein the titanium is in the form of a layerabout 20 nm thick and the titanium nitride layer is 20 nm thick.
 37. Theapparatus of claim 14, wherein the reflective switch criticaltemperature is less than a melting temperature of the workpiece butgreater than an activation temperature of the one or more processregions.
 38. The apparatus of claim 20, wherein the thermal insulatinglayer comprises two or more layers, wherein one of the layers is siliconoxynitride.
 39. A system for controlling the amount of heat transferredto one or more process regions of a workpiece, comprising: a lasersource; a workpiece support member arranged adjacent the light sourcefor supporting the workpiece; and a reflectivity switch apparatuscomprising: i) an absorber layer deposited atop the workpiece, a portionof which covers the process region or regions, wherein the absorberlayer absorbs laser radiation and converts the absorbed radiation intoheat; and ii) a reflective switch layer deposited atop the absorberlayer, wherein a portion of said reflective switch layer covering saidprocess region has a temperature that corresponds to the temperature ofthe process region and changes reflectivity from a low reflectivitystate to a high reflectivity state at a critical temperature so as tolimit the amount of radiation absorbed by the absorber layer and thusthe amount of heat transferred to the process region.
 40. The system ofclaim 39, wherein said laser source emits radiation at a wavelengthbetween 500 nm and 1100 nm.